Certain semiconductor devices have one or more blocking PN junctions in their structures. Such semiconductor devices include thyristors, transistors, diodes, diacs, triacs, reverse switching rectifiers and reverse conducting thyristors.
It has recently been demonstrated to irradiate semiconductor devices to modify the electrical characteristics in various ways. See, e.g., U.S. Pat. Nos. 3,809,582, 3,840,887, 3,852,612, 3,872,493, 3,877,997, 3,881,963, 3,881,964, 3,888,701 and 3,933,527, and U.S. Patent applications Ser. Nos. 540,208 (filed Jan. 10, 1975), 581,255 (filed May 27, 1975), and 357,435 (filed May 4, 1973), all of which are assigned to the same assignee as the present invention.
More particularly, U.S. Pat. Nos. 3,881,963 and 3,809,582 teach to irradiate high power thyristors and diodes with radiation sources generally and electron radiation sources preferably to decrease the turn-off time and reverse recovery time of such devices. Such irradiation has been demonstrated to produce distinct advantages over gold diffusion previously employed to produce fast switching devices. However, these radiation techniques have been found to have their limitations where other electrical characteristics and particularly forward voltage drop are to be maintained. If a very low turn-off time or reverse recovery time is desired, a higher forward voltage drop has to be tolerated in the device. Therefore, simply irradiating the thyristor or diode to reduce the turn-off time or reverse recovery time has involved a trade-off to a greater or lesser degree with forward voltage drop.
The limitation of this trade-off has been reduced to some degree by annealing processes described in U.S. Pat. No. 3,888,701, granted June 10, 1975. However, annealing has involved added processing steps and added time and expense in the fabrication of the semiconductor devices. And still some trade-off of turn-off time and reverse recovery time with forward voltage drop was necessary.
The present invention eliminates the need for annealing to relieve the trade-off involved in previous simple irradiation processes. It provides, by simple irradiation, high power semiconductor devices having blocking PN junctions with low switching times heretofore unattainable by simple irradiation processes while maintaining the forward voltage drop.
The nature of defect generation in silicon by proton irradiation has been investigated experimentally, Y. V. Bugakov and T. I. Kolomenskaya, Soviet Physics-Semiconductors 1, 346 (1967) and the nature of defect generation by proton, deuteron and alpha irradiation been predicted theoretically, Y. V. Bugakov and M. A, Kumakhov, Soviet Physics-Semiconductors 2, 1334 (1968). These analyses demonstrated that the defect generation by such nuclear irradiation is concentrated in relatively narrow regions near the end of the particle penetration into the silicon. FIG. 1 shows the predicted defect generation distribution in silicon for protons of 6.3 MeV energy, where curve 1 illustrates the rate of generation of defects by a single particle, curve 2 illustrates the rate of generation of defects for a beam of monoenergetic protons taking into account straggling, and curve 3 illustrates the rate of generation of defects for a beam of monoenergetic protons taking into account straggling and multiple scattering. FIG. 2 shows the experimentally measured depth dependance of resistivity (.rho.) in previously 30 ohm-cm resistivity silicon after irradiation with protons of 6.3.+-. 0.2 MeV energy, where curve 1 is to a radiation dosage of 6.8 .times. 10.sup.12 p/cm.sup.2, curve 2 is to a radiation dosage of 1.2 .times. 10.sup.12 and curve 3 is to a radiation dosage of 5.2 .times. 10.sup.11 p/cm.sup.2, and the corresponding dotted curves represent theoretically produced resistivity dependance. The analysis shows that the spatial distribution of defect generation (gaussian shaped) peaks near the end of the proton range with a narrow width at half maximum, i.e., "half-width". The analyses predicted similar spatial distribution of defect generation for alpha particles with a narrower half-width.
The analyses did not, however, disclose or suggest that such nuclear radiation defect generation could be utilized to unique advantage to improve the switching time of certain semiconductor devices.